1. Field of the Invention
The invention relates to an interactive display system of the kind having a refresh raster or matrix addressed display device and incorporating a `windowing` process by which means specified portions or `windows` of application data may be selected and transformed to be displayed in a predetermined region or `viewport` on the screen of the display device.
2. Description of the Prior Art
Such interactive display systems are well known as can be verified by reference to standard text books on the subject such as "Principles of Interactive Computer Graphics" by Newman and Sproull, 2nd Edition 1979 and "Fundamentals of Interactive Computer Graphics" by Foley and Van Damm 1982. In these text books the term `world coordinate system` is used for the space in which the picture specified by the application is defined, and the term `viewing transformation` for the transformation that converts this picture into screen coordinates. The world coordinate system is chosen to suit the application program whereas the screen coordinate system is inherent in the design of the display. The viewing transformation forms a bridge between the two and in general allows any desired scaling, rotation, and translation to be applied to the world-coordinate definition of the picture. The less general case, in which no rotation is applied by the viewing transformation is called the window transformation.
The windowing transformation is so named because it involves specifying the `window` in the world coordinate space surrounding the information required to be displayed. In addition to the `window`, a `viewport` or region on the screen in which the `window` contents are to be displayed can be defined. Generally speaking the viewport is a rectangle on the screen and may correspond to the full screen dimensions but is often considerably less. By using a viewport smaller than the full screen, room is left for other data such as menus, text messages each of which may be displayed in its own separate viewport.
In this terminology, the window is used to define what is to be displayed and the viewport specifies where on the screen it is to be displayed. Such scanning systems enable a user to perform a variety of operations, for example scanning over a large picture keeping the window size constant and varying its position with respect to the larger picture or changing the picutre magnification by changing the window size but keeping the viewport size constant. Techniques for performing these windowing transformations involving such programming devices as clipping algorithms, for example, are not regarded as forming part of the present invention and since such techniques are adequately described in the aforementioned text books and well known in the industry, detail of their implementation is not regarded as being necessary to the understanding of the present invention to be described herein, and consequently will not be given.